This issue was not related to a dangerous water leak identified in a July mission, NASA said, as it pushed the second stage of the repairs back one day to Tuesday, December 24. The earlier problem, involving water leaking into a helmet, was considered potentially life-threatening and had prompted several changes to the suits.

Veteran astronaut Rick Mastracchio's spacesuit on Saturday displayed a problem with its sublimator, a cooling unit, when he entered the ISS airlock. Instead of wearing the suit again, NASA allocated a day to resize another spacesuit for Mastracchio.

"During repressurization of the station's airlock following the spacewalk, a spacesuit configuration issue put the suit Mastracchio was wearing in question for the next excursion," NASA said in a statement.

The second spacewalk out of two - or possibly three - excursions will aim to complete an operation to replace a faulty ammonia pump module. The defective part was removed on Saturday when Mastracchio, on his seventh spacewalk, accompanied rookie Michael Hopkins in an operation completed half an hour ahead of schedule. Asked by mission control whether they could stay out longer and carry out more operations, Mastracchio declined without specifying precisely why.

Large, cumbersome component

The cooling problem, affecting the US side of the ISS, is not thought to endanger the six-person crew - but forced them to switch off non-essential equipment and interrupt several running experiments. Engineers first tried to fix the problem, identified on December 11, remotely from Earth
but decided after around a week to replace the faulty part instead.

The astronauts wore helmet cameras and NASA TV broadcast the operation in real-time from their perspective.

The cooling pump was installed in 2010 in a difficult set of spacewalks by Doug Wheelock and Tracy Caldwell Dyson; Dyson told NASA TV on Saturday that the pump was difficult to work on because of its sheer size (780 pounds or 354 kilograms, five feet or 1.5 meters in diameter) and because it contains liquid ammonia - a potential hazard if it came into contact with astronauts' suits. She also said it was difficult to focus in such exceptional surroundings.

"When you're on one of those pallets, you really have that sensation that you are sticking out on the edge of a skyscraper. Especially when you look down, you see your feet and then you see the Earth going 17,500 miles per hour (28,164 kmh) beneath you, it really does get your attention," Dyson said.